The invention relates to cage-type carriers of the type including a lower section supporting a plurality of closely spaced, parallel semiconductor wafers and having a removable cover that mates with the lower section, in effect, to provide a tube surrounding the wafers in order to achieve uniform flow of reactant gases between the wafers.
One type of quartz wafer boat or carrier that has found widespread commercial acceptance for various processes used in the semiconductor industries is referred to herein as a "cage-type" carrier. The state-of-the-art for cage-type carrier is indicated by one that is manufactured by Process Technology Ltd. of Oromocto, New Brunswick, in Canada. That cage-type carrier includes a 180.degree. semicylindrical thin quartz sheet forming a boat section having three or four grooved rods that support about 25 large semiconductor wafers in precisely spaced, parallel fashion. A similar 180.degree. semicylindrical thin quartz sheet forms a cover section that rests on the upper edges of the boat section, so that the wafers loaded in the boat section are, in effect, surrounded by a cylindrical quartz tube. Radial slots are formed in the boat section to allow outward flow of reactant gases into the cage-type carrier and thereby provide uniform gas flow patterns between all of the wafers therein.
A major problem of all of the prior cage-type carriers is that they are not easily adapted for effective use with automatic wafer transfer machines. To understand the importance of this disadvantage, it is necessary to understand that many, perhaps most major semiconductor manufacturers have moved as far in the direction of providing automated wafer handling as is presently practical, and also have taken as many steps as practical to minimize frictional contact between wafers, wafer boats, and different components of all wafer handling systems in order to minimize generation of "quartz dust" and "silicon dust". As part of this effort, semiconductor manufacturers also have attempted to completely eliminate the traditional use of metal tweezers to grip edges of wafers to transfer them from one carrier to another. The above-mentioned quartz dust and silicon dust are known to adhere, by static electricity or otherwise, to surfaces of semiconductor wafers during various manufacturing processes and thereby produce wafer defects that substantially reduce the manufacturing yield and hence substantially increase the per unit cost of the manufactured electronic chips.
A problem that exists with all prior cage-type carriers is that in order to use them in conjunction with state of-the-art automatic wafer transfer machines, it has been necessary to provide large slots in the bottom of the semicylindrical section boat of the cage-type carrier to admit the elevator members of some wafer transfer machines, such as the one described in commonly assigned U.S. Pat. No. 4,573,851. It is undesirable to provide such large slots because they interrupt the uniform reactant gas flow patterns between the wafers inside the closed cage-type wafer carrier during semiconductor processing operations. Nonuniformity of the reactant gas flow patterns between wafers causes reduction in the manufacturing yield of the chips.
Accordingly, it can be seen that there is an unmet need for an improved cage-type carrier that can be conveniently loaded and unloaded with semiconductor wafers, especially in "back-to-back" processes in which two wafers are loaded into each slot, with their back surfaces adjacent or abutting to minimize exposure of the back surfaces with reactant gases without disrupting the uniform gas flow between loaded wafers that otherwise is achievable with cage-type wafer carriers. Positioning the wafers in abutting back-to-back relationship also has the advantage of increasing the distance between the "active" or "processing" faces of adjacent wafers, because this increases gas reaction rates, such as deposition rates, on the processing surfaces of the wafers. There is a need for a cage-type carrier that can be loaded with back-to-back pairs of abutting wafers without use of tweezers.